Electrical musical instrument



J. M. HANERT ELECTRICAL MUS'ICAL INSTRUMENT March 17, 1942.

3 Sheets-Sheet 1 l Filed ont. 14, 1940 MSTER 05 C/L LA TOE F/ES T STA GE FREQUENCY DOUBLER 532 534 ffm s TW

l I l l l l 1 l |||I lrlln HIGH OCT VE 0W OCTA VE M/DDLE 067A VE f/z fe/*afar* March 17, 1942. .L M. HANERT ELECTRICAL IVIUSICALl INSTRUMENT Filed oct. 14, 1940 s sheets-sheet 3 TRIP SIGNAL FIF TH .5 TA GE FEQUENCY DOI/81.542 44 P, DSTOR TEE TRIP .SIGNAL D/STORTER FOU/2 TH 57H65 FREQUENCY DOUE/ EE Paws@ IMPL /F/Ee TONE' CONT ROLS I +/0o u EXPEESS/ON CONTKOL ity.

Patented Mar. 17, 1942 ELECTRICAL MUSICAL INSTRUMENT John M. Hanert, Wilmette, lll., assignor to Hammond Instrument Company, Chicago, Ill. corporation of Delaware Application october 14,1940, serial No. 361,064

14 Claims.

My invention relates generally to electrical musical instruments, and more particularly to instruments of this type adapted for the playing of melodies as distinguished from instruments on which chords may be played.

It is an object of my invention to provide an improved frequency generating system for a melody type electrical musical instrument.

A further object is to provide an improved frequency interlocked :cascaded oscillator system in which the frequency is successively doubled, from stage to stage, to provide a series of octave interval frequencies.

A further object is to provide an improved frequency generating system employing a plurality of frequency doubling cascaded stages, each generating a substantially saw-tooth wave shape which is musically desirable, and providing interstage coupling means which distorts the trip signal Wave into a wave shapevwhlch is substantially symmetrical about-Y its vertical axis.

A further object is to provide an improved signal output circuit for electrical musical instruments having means for suppressing the sharp peak curvature of the input wave, and thus rendering the output wave shape of smoother contour to correspond to a more mellow* tone qual- A further object is to provide an improved pushpull oscillator.

A further object is to provide a simple form of electrical musical instrument capable of rendering a melody, with or without octave coupling, which includes but a small number of parts and which may be economically manufactured.

Other objects will appear from the following description, reference being had to the accompanying drawings in which:

Figures 1, 1a and 1b together constitute a wiring diagram of the instrument;

Figure 2 is a diagram graphically illustrating wave shapes present in some of the generator circuits; and

Figure 3 is a vertical sectional view diagrammatically showing one of the playing keys and switches operated thereby.'v

General description In general, the instrument comprises three octaves of keys herein designated IC to IB, 2C to 2B, 3C to 3B which are adapted to control the transmission of signals from the generating system to the output amplifier and speaker of the instrument. Each of the keys operates three switches, the iirst of which renders the ampliler ineffective when two keys of diiferent octave "groups are simultaneously depressed, the second of which determines the tuning of the generators to the desired note frequency, and the third of which controls the operation of a relay which conby a stable master oscillator generating the frequencies of the lowest octave of the instrument. The system is so arranged that only the master oscillator frequency is controlled by the depression of the playing keys,and this change in frequency is reflected throughout the cascaded frequency doubling stages to provide corresponding changes in their frequencies.

Associated with the master oscillator is a simple vibrato apparatus by which the frequency of oscillation of the master oscillator may be varied throughout a vibrato range at vibrato periodicity, this vibrato means having manually operated controls whereby the degree of its eiect may be predetermined. This vibrato frequency shift is reflected, proportionately, in the successive frequency doubling stages. 1

The instrument is provided with controls operating in a manner similar to those disclosed in the application of Laurens Hammond and John M. Hanert, Serial No. 293,444. filed September 5, 1939, now Patent No. 2,233,258, granted December 31, 1940, whereby the outputs of several stages of the generating system may be combined at will to produce octave coupler elfects.

The output circuit of the instrument includes suitable tone control resonant circuits, and also a suitable vacuum tube apparatus for radically changing the relative amplitudes of the harmonics of the tone.

Frequency generating system.

As previously stated, the frequency generating system comprises a low frequency master oscillator and a plurality of stages of frequency multipliers which successively double the frequency generated by the master oscillator, thereby to provide the successive octave intervall frequencies. 'I'he master oscillator comprises a pair of pentodes VI and V2 which may be of the 6J7G type. These tubes are connected in' push-pull arrangement, their cathodesbeing connected to' ground through a self-biasing resistor RI and their control grids being connected to the ends of the primary winding LI of an oscillation transformer TI. The center tap of the winding LI is connected to ground. The inductance of LI and L2 may be varied, preferably by changing the air gapin the iron laminations to tune the oscillator in the first instance.

The screens of the tubes VI and V2 are connected to a terminal 300V of the power supply system having a voltage of 300 `volts positive with respect to ground, this connection being through a voltage dropping resistor R2. The suppressor grids of the tubes VI and V2 are connected to their cathodes. 'Ihe plate of tube VI is connected through a blocking condenser CI and a feedback resistor R3 to the grid of the tube V2. Similarly, the plate of the tube V2 is connected through a blocking condenser C2 and feed-back resistor R4 to the grid of tube VI. Current is` supplied to the plates of the tubes VI and V2 from terminal +300V of the power supply through resistors R5 and R6 respectively.

The push-pull alternating current output of the tubes VI and V2 is connected, through blocking condensers C3 and C4, across voltage divider resistors R6, R1, R8 and R9 which are connected in series.

Conductors II and I2 are connected respectively to the grids of the tubes VI and V2. As will more fully appear hereinafter, the capacitance between the conductors II and I2 is controlled by key operated switches, and thus such capacitance is in effect connected across the ends of the winding LI of the transformer TI to form a resonant circuit for substantially determining the frequency of oscillation of the master oscillator tubes VI and V2.

It will be noted that the plate of the tube VI feeds energy into the grid of the tube V2, and similarly the plate of the tube V2 is capable of feeding energy into the grid of the tube VI in opposed phase relation, thus fulfilling one of the conditions necessary for self oscillation.

It will be seen that the circuits are such that increased plate current through the tube VI resuits in a lowering of the potential of the grid of tube V2, thus decreasing the plate current through tube V2 and increasing the potential of the plate of said tube. This increased plate potential of the tube V2 results in increased potential of the grid of tube VI, thus further increasing the plate current through the tube VI until a further increase in grid voltage does not result in increase in plate current through the tube VI. At such time the potential of the grid of the tube V2 begins to drop and a reverse sequence of events takes place. Of course the increase and decrease of plate current through the tube V2 takes place in the same manner but 180 out of phase with respect to the corresponding changes in current'through the tube VI, the tubesVI and V2 thus producing outputs across the resistors R6 to R9 which are substantially 180 out of phase.

Vibrato apparatus A second winding L2 on the oscillation transformer TI is provided to vary the inductance of the winding LI periodically at a vibrato frequency. 'I'he meansfor accomplishing this comprises a vibratory reed I4 which is connected to ground at its fixed end and at its free end carries an iron core I6, the mass of which relative to the stiffness of the reed I4 is such as to cause the latter to have a natural frequency of vibration of approximately 'l C. P. S. The core I6 moves in and out of the field of a coil L3, one terminal of which is grounded and the other terminal of which is connected to a tap I8 of an inductance L4.

A pair of switches 20 and 2I is connected together for simultaneous manual operatior, the arrangement being such that the switches are alternately closed in such manner both switches are never closed at the same time. When the switch 20 is closed and the switch 2| open, as the Switches are illustrated in full lines in Fig. 1, one terminal of the coil L2 is connected directly to the ungrounded terminal of the coil L3 and the coil L4 is completely disconnected from the circuit so as to have no effect upon the operation. The degree of frequency shift is then dependent upon the change in inductance of L3 as the iron core I8 vibrates in its field. The amount of vibrato of course will also depend upon the number of turns of L2 relative to LI and to L3.

When the switch 20 is open and the switch 2I closed, as indicated in dotted lines in Fig. l, the ungrounded terminal -of L3 is connected to the tap I8 on L4. The tapped inductance L4 then functions as an autotransformer to reduce the effectiveness of the variable inductance L3 in producing variations in the tuning inductance LI. By so doing, the degree of vibrato is reduced. However, the average inductance (likewise the frequency of the master oscillator), looking out of the winding L2, is the same as for the vibrato on position ofthe switches 20 and 2|, because of the added shunt inductance L4. In effect, the shunt inductance of L4, which of course is not variable, compensates for the loss in effectiveness of the variable inductance L3 when the vibrato switches 20 and 2I are turned off. The position of the tap I8, together with the inductance of L4 determines the degree of residual vibrato present when the switches are in off position. It is of course desirable when playing practically all musical selections to have at least a small amount of vibrato present in the tones.

The vibrato reed may be driven through any suitable means, illustrated herein as an electromagnet 22 having one terminal thereof connect ed to a suitable source of direct current potential indicated as +V and the other terminal thereof connected to an adjustable flexible contact 24 which is adapted to be contacted by the reed as the latter swings upwardly away from the elec tromagnet 22. The reed is grounded and a spark suppressing resistor RIO is preferably connected across the contacts. It will be apparent that the reed will be maintained in vibration by the electromagnet in the manner of a vibratory interrupter. The electromagnet 22 is preferably provided with a shading ring to cause the phase of the flux change to lag the closing of the contacts.

When the above described vibrato apparatus is used in conjunction with a high frequency oscillator the core I6 is preferably made of powdered iron so as to reduce core losses.

Frequency doublers The push-pull output of the master oscillator is supplied through a pair of conductors 25 and 28, the conductor 25 being connected between the resistors R6 and Rl, and the conductor 26 being connected between the resistors R8 and R9. The conductor 25 is connected to the grid of a tube V3 through a feed-back winding L5 and a protective resistance RII. Similarly, the conductor 26 is connected to the grid of a tube V4 through a feed-back winding L6 and a protective resistance RI2. The tubes V3, V4 are preferably triodes of the 6J5G type. The cathodes of tubes V3 and V4 are connected to ground, and appropri' ate grid bias is supplied through a terminal 28 of the power supply, this grid bias being applied oi' which the inductance L5 forms a secondary winding. The other end of the primary winding L1 is connected to ground through a relatively large condenser C1 across which a signal voltage is developed, the condenser C1 being shunted by an adjustable potentiometer PI.

Similarly, the plate of the tube V4 is connected to one end of a primary winding L8 through a timing condenser C6, the winding L8 forming part of a transformer T4 of which the winding- L6 forms a secondary feed-back winding. The other end of the winding L8 is connected to ground through a relatively large condenser C8 across which a signal voltage is developed, the condenser C8 being shunted by a resistor RIS.

The tubes V3 and V4 and associated circuits .operate in the manner of two independent relaxation oscillators operating in out of phase relation by 180, and whose frequencies of relaxation are controlled by tripping signals derived from the master oscillator through the conductors 25 and 26. In operation, current ilows through theresistor RI3 to charge condenser C5,

thus gradually increasing the potential on the s plate of the tube V3.

As the condenser C5 becomes charged, current will flow through the winding L1 and condenser C1. This current flow through induction with the winding L5 will cause the potential on the grid of the tube V3 to rise because of phase arrangement of L5 with respect to L1. As the potential on the grid V3 approaches the value at which the tube V3becomes conducting, a positive trip signal from the master oscillator will be impressed upon the grid of the tube V3 through the circuit including the conductor 25, and this trip signal will, by raising the potential of the grid of the tube V3 to a value at which the tube is conducting, exactly determine the instant at which conduction through the tube commences.

As the tube V3 conducts current, condenser C5 will rapidly become .discharged and through the associated winding L1 will4 induce an opposite current in the winding L5 which will lower the potential on the grid of the tube V3 to a potential at which the tube V3 -is cut off, thus completing the relaxation cycle.

The circuit elements associated with the tube V4 are identical in value to the corresponding elements associated withthe tube V3 and thus the tube V4 will tend to oscillate at substantially the same natural frequency as the tube V3, the only difference in the natural frequency of oscillation of these two tubes being due to unavoidable irregularities in tube characteristics and in the values of the circuit elements. Since the tripping signal for the tube V4 is derived through the conductor 26, it will be 180 out of phase with respect to the tripping or locking signal supplied through the conductor 25, and as a result, the outputs derived across the signal condensers C1 and C8 will likewise be 180 out of phase.

1n order to compensate for differences in tube characteristics of V3.and V4 and other unavoidable differences in the values of circuit constants, the potentiometer PI is adjusted to such a position that the signal through a decoupling resistor RI1 to a junction terminal 30 is made identical in amplitude with the signal derived across the condenser C8 through a decoupling resistor RI8. It will thus be apparent that at the junction terminal 30 there will result a signal of generally saw-toothed wave shape of a frequency twice that of the master oscillator, while across the condenser C8 therewill be a saw-toothed wave of the same frequency as that of the master oscillator. Thus, a conductor 32 may be connected to the ungrounded terminal-of the condenser CB to provide a signal to be controlled by the key operated switches and other controls hereinafter to be described, while a conductor 34 connected to the junction terminal 30 may similarly be utilized to supply a substantially sawtoothed signal one octave higher than that provided through the conductor 32.

It will be understood that the tubes V3' and V4 together with their associated circuits above described, thus constitute a frequency multiplying, or more specically a frequency doubling apparatus, for which any other suitable frequency doubling apparatus might be substituted. For example, a full wave rectifier circuit employing either biased or unbiased tubes, a multi-vibrator circuit, a gas tube relaxation oscillator, or similar apparatus might be used. However, the alternately operating relaxation oscillators herein disclosed are vpreferred since they are not sensitive to changes in the voltages or wave shape of the trip signal throughout a considerable range, and since they produce signals of musically desirable wave shape.

Trip signal distorter The output of the tubes V3 and V4 constitutes a saw-tooth wave having a very sharp negative peak as represented by the wave A in Fig. 2. This negative peak is caused by the counter E. M. F. oiered by the inductances of the transformers T3 and T4. This wave appears at a junction terminal 3S which is connected through decoupling resistors RIS and R20 with the plates of the tubes V3 and V4 respectively.

The signal at the junction 36 is, however, not entirely satisfactory for use as a tripping signal for the next stage of frequency multiplication because of its unsymmetrical character about the vertical axis. In order to `render this wave substantially symmetrical, it is transmitted through a non-linear impedance in the form of a triode V5 whose electrodes are operating at low potential. Under these conditions, where a very large signal is applied to the control grid, the plate current will be limited on both the positive andnegative swings of the grid voltage which corresponds respectively to the point at which grid current flows and plate current cutoff occurs.

The grid of the tube V5 is connected to the terminal 36 through a blocking condenser C9 and a series grid resistor RZI which functions to limit the grid current on high positive swings of the signal. The grid is connected through the grid resistors R2| and R22 to a negative bias terminal of the power supply system. The cathode of the tube V5 is grounded while the plate, as previously indicated, is connected to a terminal of the power supply of relatively low potential indicated as a terminal |l00V of the power supply system. This connection is through a plate resistor R23 in shunt with a primary winding L9 of a coupling transformer Tl. The secondary LID of this transformer is center tapped for pushpull operation of the second stage frequency doubler tubes. The tube V5 may be of the 6J5G type.

Referring to Fig. 2, curve B represents the current wave in the plate circuit of tube V5, While curve A represents the signal voltage at terminal 36. From these curves it is seen that the plate current reaches a value at which grid current commences flowing at a very small percentage of the total positive swing of signal voltage, and that an increase of positive signal voltage above this Value produces a negligible increase in plate metry about the vertical axis is desirable for tripping the frequency multiplier tubes of the following stage in substantially 180 phase relation. This effect is improved by employing a low plate potential on tube V5, such as +100V indicated.

The second stage frequency multiplier is identical with the first stage previously described except that the values of the various circuit elements such as Rl3, R14, C1, C8 and the transformer T3 are chosen such that the second stage relaxation oscillators tend to relax at a rate approximately twice that of the rst stage frequency doubling relaxation oscillators. The signal is derived from the second stage frequency doubler through a conductor 38.

Similarly, the third and fourth stages of frequency multiplication are identical in circuit arrangement with the first stage (but having elements of suitably different values) and the tripping signal distorters between these stages are likewise similar in circuit arrangement to that between the first and second stages. The fifth stage differs slightly from the previous stages in the circuit arrangement in that the load offered by resistors RIS and R20 in the corresponding first frequency multiplicationl stage is absent, resulting in slightly lowered values for the timing resistors Rl 3 and RI 4 than would otherwise be required. The signals from the third, fourth, and fifth stages are derived through conductors 40, 42 and 44 respectively.

lt will thus be seen that the generating system comprises a low frequency master oscillator which is tunable over an octave range, and which transmits a tripping signal to a rst stage of frequency multiplication. Each of the stages of multiplication successively multiplies the frequency by a factor of two, so that six frequencies of successive octave intervals are provided. It will be understood that a change in the frequency of the master oscillator from one note to another note in its octave range is reflected throughout the frequency multiplier stages so that the frequencies generated by the latter vary proportionately with the changes in frequency of the master oscillator.

Register controlling apparatus It is desirable in a melody type instrument to provide means whereby the outputs of a plurality of stages may .be combined upon the depresl respectively to a bus bar 50 associated with the lowermost octave of keys, each through a decoupling resistor R24. Similarly, the conductors 34, 38, '40 and 42 are adapted to be connected by switches 4Gb, 41h, 48h and 49h respectively with a middle octave bus bar 52, each through a decoupling resistor R24. In the same manner conductors 38, 40, 42 and 44 are adapted to be connected by switches 46c, 41o, 48e, and 49e respectively With a high octave bus bar 54, each through a decoupling resistor R24.

Each of the bus bars 50, 52, 54 is grounded through a resistor R25 which may be of approximately the same value as the resistor R24. The impedance of each of the frequency generators is made low, in the order of 5000 ohms, and assuming this value lis the generator impedance, the resistors R24 and R25 may have a value in the order of 50,000 ohms to effect substantial decoupling between the various generators when a plurality of the tablets 46-49 are simultaneously operated.

The resistances R25 serve as a load resistor to decrease the amplitude of the signal operating upon the associated bus bar 50, 52 or 54 when a plurality of the tablets 46-49 are simultaneously operated, so that the peak amplitudes of the signals on these bus bars remain approximately the same, regardless of the number of tablets 46-49 which are simultaneously operated. In this way the full power output of the instrument as a whole may be advantageously used on any combination of tablets 46--49 without danger of overloading the amplifier and speaker.

The bus bars 50, 52, and 54 lead to relay operated switches 50a, 52a and 54a respectively and are normally in the position shown in the drawings, so as to cci-meot the bus bars 50, 52 and 54 to ground. By connecting these bus bars to ground, the outputs of the various stages through decoupling resistor R24 are grounded, and thus are prevented from undesired coupling through their high impedance series paths with other generators connected to the same bus bar. This coupling condition would otherwise prevail when more' than one of the register control tablets 46-49 are operated.

The control tablets 46-49 may bear appropriate indicia, such as Bass, Tenor, Contralto,, and Soprano respectively, to indicate the pitch registers determined thereby.

As previously indicated', and as will appear more fully hereinafter, there are three octaves of keys, and depression ofany key in the lower octave will result in energization of a relay winding 56. Similarly, depression of any key in the middle octave will result in the energization of agresse the relay winding 51, while depression of any key in the highest octave will result in energization of the relay winding 58. These relays are for the purpose of connecting the outputs of the generators to the grid of a pre-amplifier tube V6.

It will be noted that energizationof the relay winding 56 will result in the disconnection of the bus bar 50 from ground, and through the switch 50a connect this bus bar to the grid of the tube V6. Similarly, energization of the relay winding 51 will result in disconnecting ,the bus bar 524 from ground and connecting it through a resistor R26 to the grid of the tube V6. Energization of the relay Winding 58 will disconnect the bus bar 54 from ground and connect it through a resistor R21 and the resistor R26, which is in series with R21, to the grid of the tube V6. v

The signal voltage on the grid of the tube V6 is determined not only by the signal on bus bars 50, 52 and 54, but also upon the voltage divider eiect ofresistors R26 and R21 in relation to a grid resistor R28 associated with the preamplifier tube V6. Iheresistors R26 and vR21 preferably are of high impedances in the order of 350,000 ohms and 400,000 ohms respectively, while the resistor R28 may bein the order of 1 megohm.

As a result, it will be apparent that a signal on,

bus bar 50, when connected to the grid of amplifier tube V6 by contact switch 50a is substantially unaffected by the grid resistor R28, whereas signals on the bus bar 52 will be reduced substantially in a ratio of the Value of voltage Adivider resistors R28 and R26.

A signal developed on bus bar 54 will be reduced to an extent determined by the values of the resistors R26 and R21, as compared with the value of resistor R28. This signal reduction is desirable because the saw-tooth signal as developed at any of the generator conductors 32, 34, 38, 40, 42 or 44 progressively increases in amplitude as the frequency of the master oscillator is tuned lower. Compensation is then made for this increase in amplitude when changing from one octave group of playing keys to another. Thus, in playing adjacent semi-tones in different octave groups, the amplitudes of the signals impressed upon the preamplifier tube V6 will be substantially equalized. It will be noted that by making the resistors R26, R21 and R28 high with respect to the values of the decoupling resistor R24, this same equalization is effected even though a plurality of tablets 46-49 are operated at the same time.

Key switch circuits As shown in Fig. 1, there are three octaves of keys, the octaves being designated the low, middle and high octaves. The keys of the "low octave are designated IC to IB, the keys of the middle octave are designated 2C to 2B, and the keys of the high octave are designated 3C to 3B. Each of the keys, with the exception of the keys IC, 2C and 3C, sequentially operates three switches, the excepted keys operating only two switches. For example, the switches for the key ID bear the reference characters IDI, ID2, ID3 and are operated in this order, the switch IDI being closed, the switch ID2 being opened, and the switch ID3 being'closed upon depression of the key ID.

The iirst closed switches, of which the switch IDI is representative, are connected in octave groups through conductors 6I, 62 and 63 respectively, and upon operation of the associated keys are adapted to make contact with a grounded bus bar 64. The conductors 6I, 62 and 63 are connected to a common conductor 66 through resistors R3I, R32 and R33 respectively, and are for the purpose of rendering the amplifier ineffective whenever two keys in different octave groups are simultaneously depressed. This means is more fully disclosed and claimed in my co-pending application Serial No. 303,728, filed November 10, 1939, and will be further referred to hereinafter in connection with the description of the operation of the instrument.

As shown in Fig. 1, there are twelve tuning condensers designated CII) to C2I inclusive. Each of these condensers has one terminal thereof connected to the conductor I2. The condense-r CII) has its other terminal connected to the conductor II. It will be recalled that the conductors I'I and I2 are connected to the ends of the tuning inductance LI' of the master oscillator;

When all of the switches of the groups including ID2, 2D2 and 3D2 are closed, all of the condensers CIU to C2I are in parallel' across the conductors II and I2, and the master oscillator will oscillate at a frequency corresponding to the note C, for example, 65,406 C. P. S.

It will be noted that corresponding switches ID2, 2D2, 3D2 etc. of the 1ow, middle and ,high octave groups are connected in series between terminals of the condensers CIS and C26, and that all of the similar group of switches (such as IEI, -2EI, and SEI) are connected in series with each other and successively across condensers CIS to C2I. II'hus, for example, if the switch ID2 or the switches 2D2 or 3D2) is opened, the condensers CIU to CI9 will remain connected across the conductors II and I2, while only the condensers C and C2I are disconnected from these conductors. By having this series arrangement of second operated switches, and hence the-parallel arrangement of the condensers CIU to C2I, these may be of relatively small size. This is a factor of some importance in the cost of an instrument of this type where the master oscillator oscillates at low frequencies and would require relatively large condensers for its tuning if the condensers were connected in series, as shown, for example,lin my prior application, Serial ANo. 274,325, filed May 18, 1939.

Since the condensers CII! through C2I are permanently connected across the conductors I I and I2 when no keyis depressed, all generators Will operate at their "C note frequencies, and it is therefore unnecessary to provide tuning switches for the keys IC, 2C and 3C.

The third switch such as IC3 to IB3, 2C3, etc. is the last switch to be closed upon depression of y its associated key, one contact of each of these switches of the low octave being connected to a conductor 1I, one contact of each of these switches of the "middle octave being connected to a conductor 12, and corresponding switch contacts for the high octave being connected to conductor 13. These switch contacts are adapted to engage a common bus bar 14 which is connected to ground.

The conductors 1I, 12 and 13 are respectively connected to the relay windings 56, 51 'and 58, the power circuit through these relay windings being completed through a common series resistor R34 Which connects to a junction terminal 14. Current is supplied to the junction point 14 through a voltage divide1` circuit comprising resistors R35 and R36 connected between a ter- The/terminal 14 is connected to the cathodes of a pair of push-pull control tubes V1 and V8 which are preferably remote cut-oil pentodes such as the 6K7G type. It will be seen that upon depression of any playing key that the resistance of the energized relay winding plus the common seriesresistance R34 is put in shunt to ground with resistor R36 of the voltage divider R35-R36, thus reducing the potential at the junction terminal 1t. This reduction in the potential of the cathodes of control tubes V1 and V8 results in a corresponding decrease in the..eiective bias on the control grids of these tubes. This change in the cathode potential of the tubes V1 and Vt results in the removal of the cutoff bias which normally existed at the terminal 14 before the key was depressed, and thus renders these tubes capan ble of conducting the signal to the power amplin ner and speaker. Upon release of the depressed playing key and the opening of its corresponding switch, such as IC3, 3B3, etc., the voltage at the terminal 14 will gradually rise to the normal potential, at which the tubes V1 and V8 are cut oil', until terminal 14 attains the potential determined by the voltage divider resistors R and R36. The rate of change of potential is rendered gradual so as to eliminate the possibility of keying transients by virtue of the fact that the-condenser C24 is connected between the junction terminal 14 and ground, and thus limits the rate of change of potential at this terminal.

As previously described, the signals from the selected stages of the frequency generating system are impressed upon the grid of the preampliiler tube V6 when one of the relay windings 56, 51, or 59 is energized. The preamplifier tube V6 is preferably a triode of the 6F5G type, and is shown as being self-biased and supplied with plate current through a load resistor R31 connected to a terminal +300V of the power supply system. The alternating signal from the plate circuit of the tube V6 is transmitted through a blocking condenser C25 and through a decoupling resistor R38 to the grid of the tube V9. The tube V9 is preferably a remote cutoff pentode of the 6K7G type, and is operated non-linearly with a very large signal on its control grid. The screen voltage to this tube is derived from a +300V terminal of the power supply system through a voltage dropping resistor R39. A condenser C26 prevents screen degeneration and is connected between the screen and ground.

A grid resistor R40 is connected between the control grid and cathode, and a grid condenser C21 is likewise connected between the control grid and cathode of the tube V9. Condenser C21 functions both to round off the sharp positive peak of the high amplitude input signal and to accumulate bias for the operation of tube V9 on positive pulses whichdrive the grid to the point at which the grid current would otherwise commence to flow.

Plate voltage is supplied from a +300V terminal on the power supply system through a load resistor RII, the output being blocked by condenser C28 and a resistor R42 which functions as a current limiting impedance to suppress very low frequencies. A switch 16 is connected between the input and output terminals of the tube also flows through V9 and constitutes a tone quality control which may be operated at will by the musician. The opening of this'switch renders the tube V9 effective to materially decrease the amplitudes of the higher harmonic partials present in the output signal of the tube V6. It will be recalled that the signals supplied by the generating system are generally of saw-tooth character. and thus include the higher harmonics lin considerable amplitude.

Due to the fact that a very large signal is applied to the control grid of tube V S, the tube will operate non-linearly and the saw-tooth wave is of such phase that the retrace curvature of the saw-tooth (which is a very nearly vertical line), will operate in a negative direction on the control. grid characteristics of tube V9. Thus, as the input signal to the grid of V9 suddenly becomes very negative, the remote cutoff characteristics of the tube V9 will round off this sharp negative peak and thus alter the output wave to a rounded contour rather than the extremely sharp saw tooth retrace curvature impressed on its input.

The tone quality of the output is dependent not only upon the quality of the inputsignal but also upon the curvature of the remote cutoff grid characteristic. Thus, by introducing this tube, the harmonic series of the output corresponds to a more mellow tone than the harmonic series of its input. It is to be noted that this reduction in harmonic content and total alteration of the harmonic series, which will be further modified as disclosed hereinafter by the tone control networks, is made possible without the use of electrical filter networks which emphasize low frequencies. As a practical matter this is of considerable importance because an inductance-capacity resistance network capable of altering the saw-tooth tone quality to the extent which is accomplished by the mute tube V9 would be extremely complicated, and would emphasize any spurious low frequency noises to a marked degree. Such spurious low frequencies are not present in the output circuit of the tube V9. The mellowing eilect of the mute tube V9 is substantially independent of the signal frequency because it is determined nearly wholly by the geometrical parameters of the remote cutoff tubes, which are not frequency discriminative.

Whenever the mute effect is not desired, the

vswitch 16 may be closed and the output of the preamplifier tube V9 thus by-passed to the tone control networks. These networks include a plurality of normally closed series connected switches 18 to 82, each of which when opened (by the player during the course of playing a musical selection), connects the output of the preamplifier tube V6, as modified, when desired, by the mute tube V9, to ground through resonant filtering meshes of different frequency response characteristics, each mesh including either an inductance, a capacitance, or a resistance, or combinations of these elements in parallel.

The output of the preamplifier tube V6. whether oi not modified by the mute tube V9.

a primary winding LII forming part of an audio transformer T5. The secondary LIZ of the transformer T5 has its terminals connected respectively to the grids of the control tubes V1 and V8.

The mid-point 83 on the winding L|2 which forms the grid return and the point at which biasing potential is applied, is connected to a volume -control apparatus which is adapted to be operated by the player to control the degree of amplification of the tube V1 and V8. 'I'his volume control apparatus comprises a potentiometer in the form of a plurality of resistors R43 of graduated value, which are adapted to be successively connected to a conductor 84. The conductor 84 is connected to a terminal +100V of the power supply system, and is adapted to be operated by a swell pedal or other suitable hand, knee or foot operated controls.

The resistors R43 each-have one end thereof connected to a junction terminal 86 which is connected to a terminal +150V of the power supply system through a resistor R44. The resistors R43 which are connected in circuit by operation of the volume control, together with the resistance R44, form a. voltage dividing network which determines the potential upon the junction point 86. This junction point 86 is connected to the mid-point 83 of the secondary winding L12 through a large decoupling resistor R45, the mid-point 83 of the secondary winding L12 being connected to ground through a condenser C29 which serves as a means to prevent sudden changes in biasing potential, and thus prevents undesirably abrupt changes in the volume of the tones .produced as the volume control is operated.

When it is desired to have the tones sound with a slow attack, a switch 88 is closed, thereby connecting a condenser C30 between the terminals 83 and 14. The function of this condenser C30 is to temporarily maintain the control grids of V1 and V8 at a cutoff point when any one of the relays 56, 51, or 58 is operated. After a short interval of time (determined in part by the size of condenser C30), the potential upon the junction point 83 will attain a value determined by the volume control network. This imparts slow attack to the tones. When the switch 88 is open, the condenser C30 is of course ineffective to produce this result.

It will be recalled that whenever any of the playing keys is depressed, the conductor 66 is connected to ground through one of the resistors R31, R32 or R33. The conductor 66 is connected to the suppressor grids of tubes V1 and V8, and is also connected to a terminal +400V of the power supply system through a voltage divider resistor R45. A condenser C31 is connected between the conductor 66 and ground so as to -prevent sudden changes in potential on the conductor 66.

The values of the resistors R31, R32 and R33 are such with respect to the value of the resistor R45 that when any key or keys in only one octave group of keys is depressed, the potential on the suppressor grids of the tubes V1 and V8 will be lowered to a proper operating value. When, however, keys in two or more octave groups are simultaneously depressed, two ormore of the resistors R31, R32 and R33 will be connected in parallel to ground, and as a result, their combined resistance will be divided by two or three as compared with the resistances when keys in but one octave group are depressed. Under these conditions, the potential on the suppressor grids of the tubes V1 and V8 will drop to a, value beyond plate current cutoff, the condenser 31 serving to render this cutoff smooth and without a. noticeable transient.

The screens of the tubes V1 and V8 are connected to a terminal |250V of the lpower supply system, while the plates thereof are connected to a terminal +400V of the power supply system respectively through the load resistors R41 pentodes, for example of the 6F6G type. The

grids of tubes V11 and V12 are self-biased through cathode resistor R49 and are connected s to ground through grid resistors R51 and R52.

The -power output tubesVll and V12 are connected to the terminals of the primary winding L13 of output transformer T6, the latter being .provided with a center tap connected to a +300V terminal on the power supply system. The screens of tubes V11 and V12 are also connected to the +300V terminal through a voltage dropping resistor R53. A secondary winding L14 of transformer T6 is connected to the voice lcoil of a loud speaker 90.

VOperation The operation of the individual portions of the circuits employed has been set forth in connection with the detailed description of the cir suits, so that it is necessary to review only generally the method of operation of the instrument as a whole.

The instrument is played inthe manner of any melody instrument, that is, the player intentionally depresses only one key at a time although in the course of playing, especially legato, he may depress two keys simultaneously, in passing from one to another. When two keys in the same octave group are simultaneously depressed, the higher'pitch key will control the sounding of the signal because of the series arrangement of the second operated switches. If, however, two keys of different octave groups are simultaneously depressed, their first operated switches, namely,

switches corresponding to the switches IDI, will result in completing circuits through two of the resistors R31, R32 and R33 in parallel to ground, thus applying cutoff bias to the suppressor grids of the control tubes V1 and V8. There can be no condition when the instrument will produce tones of spurious pitch tones because whenever the keys are depressed in a manner which would otherwise produce tones of such spurious pitch, the output of the instrument is cut off yat the control tubes before the second operated key switch is effective to change the pitch of the generating system.

The register controls 46-49 are pre-selected by the musician to pitchthe tones of the instrument in the desired musical register. For

instance, the operation of control 46 will cause production of tones in the bass voice range, op-

eration of 41 will cause Vproduction of tones in the tenor voice range,'48 in the contralto voice range, and 49 in the soprano Voice range. Thus, the playing keys do not have any inherent pitch, but the pitches of the tones produced upon the operation of the keys is dependent upon the register controls `which are selectively operated by the player. By employing three octaves of keys, however,v it is possible to play practically any melody without having to make any changes in the register control switches 46-40. Thus, the three octave keyboard can produce tones covering a six octave range whose lowest fundamental frequency, the note Cl, is 65.406 C. P. S., and the highest fundamental frequency, the note B6, is 3951.07 C. P. S.

In many casesA the musician will operate more than one of the controls 46-48 to produce a chorus of octave tonalities similar to the effect produced by octave coupler stops of organs. Also, he will selectively operate the desired tone quality controls 18-82 by opening one or more of the switches IB- 82. Likewise he may open switch I6 which will render the mute tube circuit operative to introduce a new harmonic series. It is -to be noted that when the mute switch 'I6 is closed,

one series of tone qualities may be obtained by the proper operation of the switches 'IB-82, while when the mute switch 16 is open an. entirely different series of tone qualities may be obtained by the operation of these switches 'l8-82.

The volume of the instrument may be adjusted preferably by a knee or foot operated control to vary the over-all acoustic output of the instrument, andthe rate of attack of individual tones maybe adjusted by opening or' closing the switch 88. In addition, the player may introduce the vibrato effect at will by operation of the switches 2li- 2l The instrument has excellent frequency stability, but because it frequently will be played with other instruments whose tuning varies, as with humidity and temperature, an adjustable tuning control is provided which varies the inductance of .the oscillation transformer Tl, and thus varies the frequency of the master oscillator. By varying this single inductance, all loi. the tones of the even tempered scale may be made sharp or flat without irnpairing their octave frequency relations and their temperament.

While each stage frequency multiplier may vary in frequency generally throughout a wide range, of as much as two?. or three octaves, nevertheless their frequencies of relaxation are always controlled by the trip signal supplied by the preceding stage.

The keyboard of the instrument may be of the type adapted to be attached to a piano in a manner such as that disclosed in the patent to Laurens Hammond No. 2,203,459, and a similar type of tone cabinet may be employed, or if desired, the keyboard and tone cabinet may be combined in a unitary structure. When the keyboard is secured to the front rail of a piano, the musician may play the melody part on the electric melody instrument with the piano, and play the accompaniment upon the piano.

Because of the fact that sustained tones may be produced bythe melody instrument, musically interesting and useful results may be obtained by playing upon both instruments simultaneously.

While I have shown and described a particular embodiment of my invention it will be apparent to those skilled in the art that numerous modications and -variations may be made in the form and construction thereof, without departing from the more fundamental principles of the invention. I therefore desire, by the following claims, to include within the scope of my invention al1 such similar and modified forms of the apparatus disclosed, by which substantially the results of the invention may be obtained by substantially the same or equivalent means.

I claim:

1. In an electric musical instrument having an output circuit including an electroacoustic translating means, the combination of a plurality of playing keys, a massive oscillator, circuits completed by operation of said keys respectively for tuning said master oscillator to any one of the semi-tone frequencies of an octave, a rst stage push-pull frequency doubler having its frequency controlled by said master oscillator, a plurality a gamut of a plurality of octaves, the combination of a plurality of octaves of playing keys, a master oscillator having its frequency of oscillation controlled by said keys, a` plurality of frequency doublers connected in cascade so as to provide signals the frequencies of which are in octave relationship, means to stabilize the frequency of the lowest frequency doubler by signals derived from said master oscillator, and means for coupling the remaining frequency doublers in cascade to said lowest frequency doubler, whereby the frequency of operation of all of said frequency doubler stages will be controlled by the frequency of oscillation of said master oscillator.

3. In an electric musical instrument, the combination of a low frequency oscillator, a plurality of push-pull frequency doubling stages, means to couple said oscillator to the first of said frequency doubling stages, and means for coupling said frequency doubling stages in cascade, said last named means including a non-linear trip signal distorting mesh.

4. In an electrical musical instrument, a pushpull frequency doubler generating a generally saw-tooth output wave, a second push-pull frequency doubler, and means to couple said first frequency doubler to said second frequency doubler to cause said first doubler to control the frequency of operation of said second doubler, said coupling means comprising an electron discharge device having an input and an output circuit, said input circuit being connected to receive the output signal of said rst doubler, and the output circuit of said device being connected to transmit a trip signalto control the frequency of operation of said second doubler, said device operating to limit the amplitude of its output signal to a value much lower than the amplitude of the input signal thereof, whereby the output signal wave form of said device will be substantially symmetrical about the vertical axis despite lack of such symmetry in the signal supplied to the input circuit of said device.

5. In an electrical musical instrument having an output circuit including electroacoustic translating means, the combination of a plurality of octaves of playing keys, a master oscillator having a tuning circuit including an inductive reactance and a plurality of condensers, each having one terminal connected to a common conductor, and a switch operated by each of said playing keys, all of said switches being connected in series, with the switches associated with corresponding keys of the different octaves connected in groups across the other terminals of adjacent condensers.

6. In an electrical musical instrument, the combination of an oscillator including an inductance and a plurality of condensers forming a tuning resonant mesh determinative of the frequency of oscillation of said oscillator, said condensers each having one terminal connected to a common conductor, a plurality of octaves of playing keys, a switch operated by each of said keys, all of said switches being connected in series and being arranged in groups, each group consisting of the switches associated with the keys f translating it into tively rapid changes of corresponding notes in the different octaves,

and each group being connected acrossthe other by each of said keys to render-said amplifier in effective when two or more keys located in different octave groups are simultaneously depressed, a second switch operated by each of said keys and effective to determine the frequencies supplied by said generators, a relay for each octave of keys, a third switch associated with each of said keys for controlling the energize.- ticn of its associated relay, and means operated by said relay to connect the outputs of selected generators to said amplifier.

8. In an electrical musical instrument having an amplier and a frequency generating system including a master oscillator and a plurality of electron discharge devices having their frequencies of operation stabilized by said master'oscillator and arranged to supply signals of frequencies which bear higher octave relationships to the frequency generated by said master oscillator, the combination 'of selectively operable inga cathode, plate, and a remote cutoi grid, an input circuit for said device, an output circuit for said device, means connected to said output means for conditioning the connection of the outputs of said frequency doublers to said ampliner, means controlled by said keys for connecting said selected outputs to said amplifier, and a mute control tube receiving a signal from said amplifier and operable to alter the wave shape.

thereof, thereby to change the effective harmonic pattern of the signal produced.

9. In an electrical musical instrument in which generators of different frequencies are provided to supply output signals of generally saw-tooth wave shape, a remote cutoff mute tube having its input connected to receive signals from said generators, said mute tube being effective to alter the wave shape of the signal lreceived thereby substantially independently of the frequency, whereby the variation in tone quality of the signals produced by said mute tube will be substantially independent of the frequency of the signal.

10. In an electrical musical instrument, the combination of an electron discharge device having a cathode, plate. and a remote cutoff grid, an input circuit for said device, an output circuit for said device, means connected to said output circuit for amplifying the output of the device and l sound, and means to supply to said input circuit a signal wave having rela-l tude sufficient to operate over the maior portion bars to ground, a

in curvature andof ampliof the grid characteristic curve of said device.

whereby the signal wave said device will in the output circuit of curvature than are present in the input wave, Y

and will operate'so as to Ahave the amplitudeof its output signal change to a negligible extent with changes in the amplitude of its input signal.

11. In an electrical musical instrument, the. combination of an electron discharge device hav'- have more gradual changes in lnecting said bus 'the signal wave in the output circuit of said device will have more gradual changes in curvature than are present in the input wave, and will cperate so as to have the amplitude of its output signal change to a negligible extent with changes in the amplitude of its input signal.

12. In an electrical musical instrument having an output circuit including electroacoustic translating means, a plurality of sources of electrical impulses maintained at octave interval frequencies, each of said sources having a pair of output terminals one of which is grounded, a plurality of busbars, means including individual switches and individual decoupling resistors for selectively connecting the ungrounded terminals of said sources to said bus bars, load resistors respectively connecting said bus bars/to ground, normally closed switches connecting said bus bars to ground, and key controlled means 'for opening said last named switches and connecting said bus bars to the output circuit of the instrument.

13. In an electrical musical -instrument having an output circuit including electroacoustic translating means, a plurality of sources of electrical impulses maintained at octave interval frequencies, each of said sources having a pair of output terminals one of which is of bus bars, means includin individual switches and individual decoupling resistors for selectively connecting the ungrounded terminals of said sources to said bus bars, load resistors respectively connecting said bus bars to ground, normally closed switches connecting said bus voltage divider network having a plurality of terminals and determining the etfectlve impedance looking into the output circuit of the instrument, and key controlled means for opening said normally closed switches and conbars to different terminals of said network, thereby to compensate for variations in the amplitudes of the signals on said bus bars with variations in the frequencies or said sources.

14. The combination set forth in claim 12 in which the impedances'oi said decoupling resistors are each in the order of ten times as great as the impedances of each of said sources, and in which the eiective impedance looking into the output circuit ol.' the instrument is in the order of twenty to forty times the impedance of each of said decoupling reslstors.

rounded, a plurality 

